A switching power supply, for example, a pulse-width modulated power supply can include an output stage having a primary winding of an isolation transformer. The transformer primary winding is coupled to a mains supply voltage in a non-isolated manner. An isolated power supply voltage is produced from a voltage developed in a secondary winding of the isolation transformer. Typically, a snubber capacitor is coupled to the primary winding for reducing the rate of change of a flyback voltage portion of a voltage developed at the primary winding.
An integrated circuit (IC) forming a pulse-width modulator controls the switching operation of the output stage. For its operation, this IC needs to be powered by an auxiliary, non-isolated power supply voltage. The non-isolated power supply voltage can be produced in a power supply that needs to provide a relatively low power. It may be desirable to produce the non-isolated power supply voltage from a non isolated voltage produced already in the primary winding of the transformer.
In carrying out an aspect of the invention, a snubber capacitor that is coupled to the primary winding is used for producing a capacitive coupled charging current. The capacitive-coupled charging current is coupled to a filter or charge storage second capacitor for producing in the second capacitor a first portion of the non-isolated power supply voltage for energizing the IC.
In carrying out another aspect of the invention, during a portion of a switching cycle of the output stage, the snubber capacitor is coupled to an inductor to form a resonant circuit. The resonant circuit produces in the second capacitor a second portion of the non isolated power supply voltage for energizing the IC.
In carrying out a further inventive feature, a switching, shunt transistor regulates the low voltage power supply voltage by selectively interrupting a current that would otherwise charges the second capacitor.